Over the past several years, X-ray baggage inspection systems have evolved from simple X-ray imaging systems that were completely dependent on interpretation by an operator to more sophisticated automatic systems that can automatically recognize certain types of contraband. The more sophisticated inspection systems have employed single energy or dual energy X-ray radiation transmitted through the examined baggage. Some systems have used a single view source detector arrangement, others have utilized a dual view or multi-view arrangements. The single or dual view systems usually scan baggage, as it moves on a conveyor, using a fan beam or a scanning pencil beam of X-rays to provide projection images. The multiview, CT type systems generally scan stationary baggage and process data corresponding to absorption of X-rays to reconstruct a cross-sectional view of the contents of the baggage. These systems usually display their decision by highlighting objects or regions in the examined baggage in different colors.
The dual energy inspection systems may also use the ratio of the attenuation values at the two energies to discriminate between low Z materials (e.g., plastics) and high Z materials (e.g., metals). However, many inspection systems cannot distinguish contraband when covered by a high density material, such as a sheet of metal.
To locate a low density material in baggage, some inspection systems employ both transmitted and scattered radiation. Such systems use an X-ray source located on one side of an inspection region and a detector located on the other side of the inspection region. The X-ray source emits a fan beam or a scanning pencil beam of X-rays that are transmitted through the examined baggage and detected by the detector. These systems may also employ an X-ray forward-scatter detector disposed near the X-ray transmission detector to detect photons scattered by the illuminated object out of the path of the transmitted beam. A back-scatter detector is usually disposed on the same side as the X-ray source and detects photons back-scattered out of the beam path by the object. The systems may display a transmission image and both scatter images for an operator, who then decides whether contraband is located in the baggage based on the shape and location of the imaged items. These systems may also digitally manipulate the scatter image and produce a selected parameter associated with a particular contraband, e.g., a histogram that is then compared to a predetermined characteristic of the probed contraband. The systems can also sound an alarm if the contraband is detected.
In general, many X-ray transmission systems are not able to effectively detect low Z materials (such as plastics or plastic explosives), especially when shaped into objects of thin cross-section, since they cause relatively small attenuation of X-rays. On the other hand, some X-ray scatter systems are not able to consistently identify weapons, explosives or drugs located deep inside baggage. Some X-ray systems have too low a throughput for use as an in-line inspection device at the airport. There is a need for a high speed X-ray inspection device that can reliably detect weapons, various explosives (or other contraband) having different shapes and sizes and being located anywhere in the examined baggage.